Implantable medical lead

ABSTRACT

An implantable lead has a lead body construction designed to accommodate loading forces exerted on the lead body during patient movement. The lead body described herein is sufficiently stretchable to resist forces that could otherwise cause lead failure, axial migration of the electrodes, anchor damage, or tissue damage. The lead body may include a variety of features that reduce the axial stiffness of the lead without significantly impacting the operation and structural integrity of lead components, such as electrodes, conductors and insulators.

This application claims the benefit of U.S. provisional application No.60/621,018, filed Oct. 21, 2004, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to implantable medical devices and, moreparticularly, implantable medical leads.

BACKGROUND

A variety of implantable medical devices (IMDs) are available to monitorphysiological conditions within a patient, deliver therapy to a patient,or both. Typically, an IMD is coupled to one or more implantable leadsthat carry electrodes to sense physiological electrical activity ordeliver electrical stimulation. Cardiac pacemakers andcardioverter-defibrillators, for example, are coupled to one or moreintravenous or epicardial leads that include sensing electrodes to sensecardiac electrical activity, stimulation electrodes to deliver pacing,cardioversion or defibrillation pulses, or a combination of sensing andstimulation electrodes.

Neurostimulation systems also include implantable leads for delivery ofneurostimulation therapy to patients to treat a variety of symptoms orconditions such as chronic pain, tremor, Parkinson's disease, multiplesclerosis, spinal cord injury, cerebral palsy, amyotrophic lateralsclerosis, dystonia, torticollis, epilepsy, urinary incontinence, fecalincontinence, sexual dysfunction, obesity, or gastroparesis or othergastric mobility disorders. An implantable neurostimulator deliverselectrical stimulation pulses via electrodes carried by leads implantedproximate to the spinal cord, pelvic nerves, stomach, orgastrointestinal tract, or within the cranium of a patient, e.g., fordeep brain stimulation or occipital nerve stimulation.

As a patient implanted with an IMD moves, some regions of the body mayexpand and contract, resulting in changes in length. The movement mayexert high loading forces on anchors, leads, lead extensions, or bodytissue. These forces may cause lead failure, axial migration ofelectrodes, anchor damage, or tissue damage. The patient may experiencepain or operational failure or performance degradation of the IMD.

SUMMARY

In general, the invention is directed to implantable leads with a leadbody construction designed to accommodate loading forces exerted on thelead body during patient movement. The lead body described in thisdisclosure is sufficiently stretchable to resist forces that couldotherwise cause lead failure, axial migration of the electrodes, anchordamage, or tissue damage. The lead body may include a variety offeatures that reduce the axial stiffness of the lead withoutsignificantly impacting the operation and structural integrity of leadcomponents, such as electrodes, conductors and insulators.

Several embodiments of a lead are described in this disclosure. Forexample, a lead body may comprise a low durometer outer jacket and/orconductors with a low modulus of elasticity, providing increasedstretchability. Increasing stretchability of a lead body can alsoincrease the vulnerability of the lead body to flex fatigue, bucklingfatigue, kinking, and crush. Therefore, in some embodiments, the leadmay also include a coiled wire stylet guide to provide enhanced columnstrength. The coiled wire stylet guide may or may not be electricallyconductive.

A helical reinforcement also may be added to the lead to create a leadbody that is resistant to flex fatigue, buckling fatigue, kinking andcrush. Furthermore, a coiled wire may be embedded between a firstinsulative layer and a second insulative layer of an outer jacket of thelead body to improve column stiffness and kink resistance. Utilizing oneor more of the above features, the lead is able to accommodate changesin length associated with typical patient movement while maintainingstructural integrity.

In one embodiment, the invention is directed to a medical lead for usewith an implantable medical device, the medical lead comprising a leadbody having a proximal end and a distal end, one or more electrodes atthe distal end of the lead body, one or more electrical contacts at theproximal end of the lead body, and one or more conductors electricallycoupling the electrodes and the electrical contacts, wherein lead bodyhas an axial stiffness of no greater than approximately 0.35 kg/cm/cm.

In another embodiment, the invention is directed to an implantablemedical device comprising a housing, an implantable pulse generator,within the housing, that generates electrical stimulation pulses, and amedical lead, extending from the housing. The medical lead comprises alead body having a proximal end and a distal end, one or more electrodesat the distal end of the lead body, one or more electrical contacts atthe proximal end of the lead body, and one or more conductorselectrically coupling the electrodes and the electrical contacts,wherein the lead body has an axial stiffness of no greater thanapproximately 0.35 kg/cm/cm.

The invention also contemplates methods of use and fabrication of animplantable lead and implantable medical device.

The invention may be capable of providing one or more advantages. Forexample, a lead constructed in accordance with the invention may resultin reduced mechanical loading on tissue anchor points, implantable leadextensions, the implantable lead itself, and the IMD during typicalpatient movement. In addition, the lead may improve resistance to flexfatigue, buckling fatigue, kinking, and crush. These features may alsoprovide advantages beyond strengthening the lead. For example, a coiledwire stylet guide may provide improved column steerability as well asenhanced stylet insertion and withdrawal. In some embodiments, straightwire conductors may be combined with the coiled stylet guide to achievelow conductor impedance while maintaining stylet maneuverability withinthe coiled guide.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a stimulation lead introducing kit,which includes components for percutaneously implanting a stimulationlead.

FIG. 2 is a schematic diagram illustrating a cutaway view of animplantable medical lead for use with an implantable medical deviceaccording to an embodiment of the invention.

FIG. 3 is a schematic diagram illustrating a cutaway view of anotherimplantable medical lead according to an embodiment of the invention.

FIGS. 4A-4E are schematic diagrams illustrating exemplarycross-sectional views of leads with axially positioned conductors.

FIG. 5 is a schematic diagram illustrating another exemplarycross-sectional view of a lead with axially oriented coiled wireconductors.

FIG. 6 is a schematic diagram illustrating a cutaway view of a coiledwire conductor.

FIG. 7 is a schematic diagram illustrating a cutaway view of anotherimplantable medical lead according to an embodiment of the invention.

FIG. 8 is a schematic diagram illustrating a cutaway view of anotherimplantable medical lead according to an embodiment of the invention.

FIG. 9 is a schematic diagram illustrating a cutaway view of anotherimplantable medical lead according to an embodiment of the invention.

FIG. 10 is a schematic diagram illustrating an implantable medicaldevice for delivering electrical stimulation pulses to a patient.

FIG. 11 is a block diagram illustrating components within the device ofFIG. 10.

DETAILED DESCRIPTION

FIG. 1 is a diagram illustrating a stimulation lead introducing kit 10,which includes components for percutaneously implanting a stimulationlead in accordance with the invention. In other embodiments, the leadmay be surgically implanted. As shown in FIG. 1, kit 10 includes aneedle 12, a needle stylet 14, a guidewire 16, a dilator 18, a sheath20, a stimulation lead 22, and a lead stylet 24. Lead 22 has a lead bodythat is constructed to accommodate loading forces exerted on the leadduring patient movement. FIG. 1 shows a distal portion 22A and aproximal portion 22B of lead 22. In some embodiments, lead 22 may besufficiently stretchable to resist forces that could otherwise causelead failure, axial migration of the electrodes, anchor damage, ortissue damage.

Lead 22 may be coupled to an implantable medical device (IMD), eitherdirectly or via a lead extension. As a patient moves, portions of thepatient's body in which an IMD may be implanted change in length. Forexample, the fascial surface dorsal to the lumbar spine elongatesapproximately 3.7 inches (9.4 cm) in a typical individual from a neutralor standing position to a fully flexed or bent over position as measuredfrom the iliac crest to the area near the spinous process of the firstlumbar vertebra. Conventional lead bodies are unable to accommodate somechanges in length within a patient's body, even with the addition ofsubcutaneous strain relief loops in the lead and/or lead extensions.Consequently, some lead bodies may be prone to lead failure orperformance degradation due to fractures or electrical shorting, axialmigration of electrodes coupled to the lead body, anchor damage, and/ortissue damage at anchor points.

Lead 22 may include a lead body constructed to exhibit a reduced axialstiffness that permits the lead body to better accommodate changes inlength along a patient's body. In one embodiment, for example, the leadbody of lead 22 exhibits an axial stiffness of no greater than 5.0pounds/inch/inch (0.35 kg/cm/cm), more preferably between approximately5.0 pounds/inch/inch and 1.5 pounds/inch/inch (0.105 kg/cm/cm), and evenmore preferably between approximately 3.3 pounds/inch/inch (0.23kg/cm/cm) and 1.5 pounds/inch/inch. These ranges of axial stiffness maybe achieved by selection of appropriate materials and design featuresfor lead 22. For example, in some embodiments, lead 22 may combine lowdurometer outer jacket materials with structural features such as lowfilar count coiled conductors to enhance stretchability, while alsoincorporating additional structural features such as a coiled styletguide and helical reinforcement wires for structural integrity.

A reduced axial stiffness in the above range promotes increasedstretchability in the lead body to better accommodate changes in lengthalong the patient's body. A medical lead body with an axial stiffness inthe above ranges may permit an axial elongation of approximately fivepercent to approximately thirty percent, and more preferablyapproximately ten percent to thirty percent, without breakage ordegradation of performance. In some cases, the enhanced stretchabilitymay substantially eliminate lead failure due to fractures or electricalshorting, axial migration of electrodes coupled to the lead body, anchordamage, and/or tissue damage at anchor points. The above axial stiffnessvalues are expressed in pounds/inch/inch, rather than simplypounds/inch, as the lead body may have different lengths, depending uponthe model and manufacturer, as well as different degrees of elongationduring use. As an example, however, the lead body of lead 22 maygenerally correspond to a lead body with a length of approximately 12inches to 14 inches (30 cm to 36 cm), and more preferably approximately13 inches (33 cm), at an elongation of approximately 1 inch (2.54 cm).In some embodiments, the lead body of lead 22 may have longer lengths,e.g., for application in which no lead extension is used to couple to anIMD. In these cases, the lead body may be up to approximately 120 cm inlength.

Several embodiments of leads are described herein. For example, a leadbody may comprise a low durometer outer jacket and/or conductors with alow modulus of elasticity. In addition, the lead may comprise a coiledwire stylet guide to provide enhanced column strength and steerabilitywhile improving stylet insertion and withdrawal. The lead may alsoinclude a helical reinforcement wire to create a lead body that isresistant to flex fatigue, buckling fatigue, axial displacement, kinkingand crush. Furthermore, a coiled wire may be embedded between a firstinsulative layer and a second insulative layer within an outer jacket ofthe lead body to improve column stiffness and kink resistance. In thisway, the lead is able to accommodate changes in length within the bodyassociated with typical patient movement while maintaining structuralintegrity. Some of the features described herein may be applied not onlyto leads, but also leads that are not significantly stretchable.

With further reference to FIG. 1, a stimulation lead 22 may bepercutaneously implanted in the epidural region proximate a spine of apatient. Although kit 10 depicts the deployment of a lead for purposesof spinal cord neurostimulation, other applications are contemplated.For example, a lead as described herein may be used in a variety ofsensing and therapy applications such as spinal cord neurostimulation,sacral neurostimulation, deep brain stimulation, and cardiac sensing andstimulation, e.g., for pacing, cardioversion or defibrillation. However,spinal cord neurostimulation will be described for purposes ofillustration.

The elements in kit 10 are not necessarily shown to scale in FIG. 1. Thediagram of FIG. 1 depicts the distal ends and proximal ends of the partsin kit 10 at the left and right, respectively. In general, a “distal”end will refer to the first end of a component that is introduced intothe patient, whereas the “proximal” end generally extends outside of thebody for manipulation by medical personnel.

Needle 12 has a lumen to receive needle stylet 14. In some instances,needle 12 may take the form of a modified Tuohy needle, which has anopening that is angled, e.g., approximately 45 degrees, so that aninstrument passing through the needle exits through the needle at anangle. Needle stylet 14 fills the lumen of needle 12 to prevent coringin the tissue of a patient when needle 12 is inserted into the patient.

Guidewire 16 is an elongated, flexible instrument that is steerable topermit deployment of the guidewire to a desired “target” site, e.g.,within the epidural region. In practice, guidewire 16 may be insertedthrough needle 12 and steered through the epidural region to the targetsite for neurostimulation therapy. Guidewire 16 prepares a path so thata stimulation lead introducer, formed by dilator 18 and sheath 20, canreach the target site by advancing over guidewire 16.

Dilator 18 has a cross-section that produces a widened path through bodytissue for deployment of stimulation lead 22. Sheath 20 fits overdilator 18 to form the stimulation lead introducer. In particular,sheath 20 permits passage of stimulation lead 22 when dilator 18 is notpresent in sheath 20, i.e., upon withdrawal of dilator 18.

Stimulation lead 22 may include a cylindrical structure with at leastone ring electrode 36 to provide stimulation to tissue within a patient,as shown in FIG. 1. In other embodiments, the stimulation lead maycomprise a paddle lead. FIG. 1 depicts a distal end of stimulation lead22, including a lead body 33, which carries electrodes 36 that functionas tissue-stimulating electrodes. A proximal end of lead body 33 iscoupled to an implantable medical device (IMD) (not shown), such as aneurostimulator that generates neurostimulation energy for delivery viaelectrodes 36. In particular, proximal portion 22B of lead 22 includeselectrical contacts 39 for electrical contact with terminals within anIMD.

Lead 22 defines a lumen that receives lead stylet 24. Lead stylet 24 maycomprise a wire sized to fit within a stylet lumen of lead 22. In someembodiments, lead stylet 24 may have an outer diameter of approximately0.012 inches to 0.010 inches (0.03 cm to 0.025 cm). Lead stylet 24 maybe substantially steerable to permit deployment of stimulation lead 22to a desired “target” site within the epidural region. In practice, leadstylet 24 may be inserted through lead 22 to steer lead 22 to the targetsite for neurostimulation therapy.

In some embodiments, lead 22 may have a length of approximately 12 to 14inches (30 to 36 cm), and more preferably approximately 13 inches (33cm). At an elongation of approximately 1 inch (2.54 cm), lead body 33exhibits an axial stiffness of no greater than 0.50 pounds/inch (0.09kg/cm), more preferably between approximately 0.5 pounds/inch and 0.15pounds/inch (0.03 kg/cm), and even more preferably between approximately0.33 pounds/inch (0.06 kg/cm) and 0.15 pounds/inch. In this manner, lead22 allows typical patient movement without causing lead failure orperformance degradation due to axial migration, anchor damage, and/ortissue damage at anchor points.

The distal portion 22A of stimulation lead 22 shown in FIG. 1 includesfour ring electrodes 36 and a spacer 37 placed between electrodes 36. Asimilar arrangement may be provided in proximal portion 22B withelectrical contacts 39. Electrodes 36 may be formed from a variety ofelectrically conductive, biocompatible materials. Example electrodematerials includes platinum and platinum iridium. Spacer 37 may comprisea polyurethane or silicone material, or an alloy of silicone andpolyurethane. In various embodiments, stimulation lead 22 may take theform of an octad lead including eight ring electrodes or a quad leadincluding four ring electrodes, shown in FIG. 1. However, stimulationlead 22 may be designed to accommodate any number of electrodes. A lineof neurostimulation leads utilizing ring electrodes is commerciallyavailable from Medtronic, Inc. of Minneapolis, Minn.

Lead body 33 of lead 22 may comprise an outer jacket 34. Lead body 33carries conductors 35 within a lumen created by outer jacket 34.Conductors 35 connect electrodes 36 to the IMD coupled to the proximalend of lead body 33. As shown in FIG. 1, a set of distaltissue-stimulating electrodes 36 in distal portion 22A are coupled to aset of proximal electrical contacts 39 in proximal portion 22B viaconductors 35. Distal electrodes 36 deliver electrical stimulationpulses to tissue within the patient. Proximal contacts 39 are coupled toan implantable pulse generator (IPG) within the IMD to receive thestimulation pulses. As an example, conductors 35 may comprise braidedstrand wire (BSW) cables.

The stranded wire used to create the BSW cables for conductors 35 maycomprise a silver core. As an example, the stranded wire may compriseMP35N™ alloy, which is a biocompatible, nonmagnetic,nickel-cobalt-chromium-molybdenum alloy with high strength and corrosionresistance, with a silver core to improve conductance. However, thesilver may create difficulties when welding conductors 35 to electrodes36, which may comprise platinum iridium (PtIr).

As one solution, a crimp tube 38 comprising a weldable material may becrimped onto the end of each of conductors 35. Crimp tube 38 may then belaser welded to electrodes 36 at distal lead portion 22A and proximallead portion 22B. Crimp tube 38 may comprise a material thatsubstantially eliminates silver from the weld. In some cases, crimp tube38 may comprise platinum. A similar arrangement may be used forelectrical contacts 39. In other embodiments, a variety of othersolutions may be utilized to connect conductors 35 to electrodes 36.

Outer jacket 34 of lead body 33 may be made of an extruded or moldedmaterial, such as a polyurethane or silicone material, or an alloy ofsilicone and polyurethane. The material may include a substantially lowdurometer material, substantially similar to an elastomer, toaccommodate changes in length within a patient's body. For purposes ofillustration, in one exemplary embodiment, assuming lead 22 isapproximately 13 in (33 cm) in length and at an elongation ofapproximately 1 inch (2.54 cm), the material of outer jacket 34 may havea modulus of elasticity between approximately 0.37 pounds/inch² (0.026kg/cm²) and 0.1 pounds/inch² (0.007 kg/cm²), and more preferably betweenapproximately 0.2 pounds/inch² (0.014 kg/cm²) and 0.1 pounds/inch².

Conductors 35, within lead 22 conforming to the above listed dimensions,may comprise an axial stiffness between approximately 0.13 pounds/inch(0.023 kg/cm) and 0.05 pounds/inch (0.009 kg/cm), and more preferablybetween approximately 0.08 pounds/inch (0.014 kg/cm) and 0.05pounds/inch. Conductors 35 comprising BSW cables may provide increasedflexibility. Coiling or helically winding conductors 35 allowsconductors 35 to elongate or stretch. In particular, the individualcoils tend to narrow in diameter as they are stretched along thelongitudinal axis of lead 22. Furthermore, the coiled or helically woundconductors may form a lumen for insertion and withdrawal of lead stylet24. To increase an overall elasticity of lead body 33, conductors 35 maycomprise a low number of filars per coil. With a low number of filars,e.g., two to four per coil, concentric conductor coils can be used toachieve a required number of conductors. Furthermore, conductor coilsmay be designed with a high coil diameter to wire diameter ratio. Inother embodiments, the conductors may comprise flat wire wound intocoils.

When lead 22 is implanted within a patient, outer jacket 34 of lead body33 may become hydrated by bodily fluids. This can alter physicalproperties of a material comprising outer jacket 34, such as apolyurethane material. Outer jacket 34 may become more stretchable whenin the hydrated state. The altered physical properties may includemodulus of elasticity, durometer, impact resistance, and the like.

Enhancing the elasticity of lead body 33 reduces forces on lead 22, leadextensions, anchors, and body tissue at anchor sites, which can causethe patient pain and/or render the IMD inoperable. In either case, notaccommodating changes in length within the patient's body can bedetrimental to the patient's health. However, increasing stretchabilityof lead body 33 can also increase the lead body's vulnerability to flexfatigue, buckling fatigue, kinking, and crush.

In order to maintain structural integrity of lead body 33 while reducingoverall axial stiffness, one or more reinforcing structures may be addedto lead 22. For example, coiled wire may form an inner stylet guidetube. The coiled wire stylet guide may be electrically conductive ornonconductive, and increases column strength and resistance to kinkingwhile providing a smooth reliable path for lead stylet 24.

A reinforcement wire may be helically wound around conductors 35 toprevent bi-lateral collapse of lead body 33 during buckling. In somecases, the helically wound reinforcement wire may create a helicalchannel in which conductors 35 may lie. Furthermore, a coiled wire maybe included within outer jacket 34 or between two thin jacket extrusionsexternal to outer jacket 34. The embedded wire may provide protectionagainst kinking of lead body 33, in a manner similar to the wire oftenembedded in a vacuum cleaner hose.

FIG. 2 is a schematic diagram illustrating a cutaway view an implantablemedical lead 40 for use with an implantable medical device according toan embodiment of the invention. Lead 40 may comprise a stretchable leadsubstantially similar to lead 22 (FIG. 1). Accordingly, lead 40 may bepercutaneously implanted using a stimulation lead introducing kitsubstantially similar to kit 10 illustrated in FIG. 1. Lead 40 mayinclude at least one electrode to provide stimulation to a patient. Theelectrode may include a ring electrode or an arrangement of electrodeson a paddle lead.

Lead 40 includes an outer jacket 42 and a coiled stylet guide 46positioned within a lumen formed by outer jacket 42. Coiled stylet guide46 may be formed by flat or cylindrical wires, which may be electricallyconductive or nonconductive. Outer jacket 42 may comprise an externaldiameter of approximately 0.045 to 0.055 inches (0.114 to 0.14 cm), andmore preferably approximately 0.052 inches (0.13 cm). Stylet guide 46may comprise an external diameter of approximately 0.012 to 0.020 inches(0.03 to 0.05 cm), and more preferably approximately 0.016 inches (0.04cm). A set of conductors 45 wraps around stylet guide 46 to form one ormore conductor coils 44. In the illustrated embodiment, lead 40comprises an octad lead with eight conductors included in set ofconductors 45. In other embodiments, lead 40 may comprise a quad leadincluding four electrodes or another type of lead including any numberof electrodes.

In some embodiments, lead 40 provides enhanced stretchability to preventlead failure, axial migration, anchor damage, and/or tissue damage atanchor points during typical patient movement. Outer jacket 42 may bemade of an extruded or molded material, e.g., a polyurethane material,with a substantially low durometer. Conductors 45 may comprise braidedstrand wire (BSW) cables that provide increased flexibility.

Conductors 45 may be constructed as BSW cables wound into a helix.Coiling or helically winding conductors 45 into conductor coil 44 allowsconductors 45 to elongate or stretch as lead 40 experiences axialloading forces during use. Helically wound conductors 45 may providedesirable axial compliance as well as needed bend-flex fatigue life. Ina case of severe buckling, the helically wound conductors 45 maycollapse, binding the conductors and concentrating the bend into a smallradius. To address this problem, as described above, a reinforcementwire may be helically wound with the wound conductors 45 in a way thatprevents bilateral collapse of the structure during buckling. The woundreinforcement wire also may be helically extruded, forming a helicalchannel in which the conductors reside, as will be described in greaterdetail herein.

Coiled wire stylet guide 46 creates a lumen 47 to receive a stylet 48.In some cases, stylet 48 comprises a wire with a diameter betweenapproximately 0.012 inches and 0.01 inches (0.03 cm and 0.025 cm).Stylet 48 may be inserted into lumen 47 of stylet guide 46 to steer lead40 to a target site within a patient's body. The coil design of styletguide 46 eases the insertion and withdrawal of stylet 48 by forming asmooth path along which stylet 48 slides. In addition, coiled wirestylet guide 46 may enhance steerability of lead 40, which increasesaccuracy when positioning lead 40 within a patient.

At a distal end of lead 40, not shown, stylet guide 46 may be sealedsuch that stylet 48 cannot extend beyond the distal end of lead 40.Sealing the distal end of stylet guide 46 decreases the probability ofinadvertently puncturing epidural tissue and causing a “wet tap,” orcerebral spinal fluid (CSF) leak, which is an event that may causesevere headaches or, if the leak is severe, may cause neurologicaldamage. A CSF leak may occur if stylet 48 extends beyond stylet guide 46into the epidural region proximate the spine of a patient, causing apuncture in the dura membrane of the epidural region.

During typical patient movement, the lead 40 may experience compressivebuckling. Conventional leads may comprise an extruded plastic styletguide. In that case, the plastic stylet guide may be prone to bi-lateralcollapse, which creates a flat and wide cross-section. If the kinkformed in the plastic stylet guide forces the conductors into a sharpbend radius, this cyclical loading may cause lead failure. In contrast,coiled stylet guide 46 can resist such problems.

FIG. 2 illustrates a coiled wire stylet guide 46 comprising anelectrically passive helically wound wire. In the illustratedembodiment, coiled stylet guide 46 comprises a flat or ribbon wire. Inother embodiments, coiled stylet guide 46 may comprise a round wire or awire with a rectangular cross section. Stylet guide 46 may comprise ametal wire, such as an MP35N wire. In some embodiments, stylet guide 46may be insulated with a polymer, such as ethylene-tetrafluoroethylene(ETFE). Other examples of insulative materials includepolytetrafluoroethylene (PTFE), modified PTFE, and polyimide, as well aspolyurethane, silicone, and polyester. Although the wire in stylet guide46 may be electrically inactive, insulating the coiled wire stylet guide46 reduces abrasion with conductors 45.

The wire is wound in a helical fashion to form a substantiallycylindrical shape for stylet guide 46. As discussed above, stylet guide46 comprises a diameter of approximately 0.012 inches to 0.020 inches(0.03 cm and 0.05 cm), and preferably approximately 0.016 inches (0.04cm). In general, stylet guide 46 comprises a diameter small enough toallow conductor coil 44 to fit between coiled stylet guide 46 and outerjacket 42 and large enough to resist crushing and collapse.

The helically coiled structure separates between adjacent turns to allowstylet guide 46 to bend, either at a corner or during compression, whilemaintaining a substantially round cross-section. Stylet guide tube 46 iscoiled in an opposite direction of conductors 45. This may preventconductors 45 from being pinched by coils of stylet guide tube 46.Coiled wire stylet guide 46 is able to substantially withstand crushingand collapse by preventing cross-sectional flattening and forcing alarger bend radius than traditional plastic stylet guides. In someembodiments, coiled wire stylet guide 46 may comprise a single wirestrand, i.e., a mono-filar cable. In this case, stylet guide 46 mayexperience less torsional stress during bending than a multi-filarcable.

FIG. 3 is a schematic diagram illustrating a cutaway view of anotherimplantable medical lead 50 for use with an implantable medical deviceaccording to an embodiment of the invention. Lead 50 may besubstantially similar to lead 22 from FIG. 1 and lead 40 from FIG. 2.Lead 50 includes an outer jacket 52 and a coiled wire stylet guide 56positioned within a lumen formed by outer jacket 52. Coiled wire styletguide 56 creates a lumen 57 to receive a stylet 58. Outer jacket 52 andstylet guide 56 may comprise diameters substantially similar to outerjacket 42 and stylet guide 46 described in reference to FIG. 2. A set ofconductors 55 lies axial to stylet guide 56, also within the lumenformed by outer jacket 52. In the illustrated embodiment, lead 50comprises an octad lead with eight conductors included in set ofconductors 55. In other embodiments, lead 50 may comprise a quad leadincluding four electrodes or another type of lead including any numberof electrodes.

As in the example of FIG. 2, conductors 55 may comprise braided strandwire (BSW) cables. As an example, the stranded wire may comprise MP25Nalloy. In the illustrated embodiment of FIG. 3, however, conductors 55comprise straight wires that extend axially along the length of lead 50.The straight orientation of conductors 55 serves to reduce the overalllength of the conductors, relative to coiled conductors, and therebyreduces conductor impedance. Decreasing impedance of conductors 55 maysignificantly increase battery longevity of an IMD to which lead 50 iscoupled.

FIGS. 4A-4E are schematic diagrams illustrating exemplarycross-sectional views of leads with axially positioned conductors. Eachof the illustrated leads in FIGS. 4A-4E may be substantially similar tolead 50 from FIG. 3. In the illustrated embodiments, the leads compriseoctad leads that include eight conductors. In other embodiments, each ofthe leads may comprise a quad lead including four conductors or anothertype of lead comprising any number of conductors.

FIG. 4A illustrates a lead 60 comprising a coiled wire stylet guide 62substantially similar to coiled wire stylet guide 46 (FIG. 2) and coiledwire stylet guide 56 (FIG. 3). In the illustrated embodiment, anelectrically nonconductive ribbon wire forms coiled wire stylet guide62. The ribbon wire may be formed from metallic alloys such as MP25-N,stainless steel, titanium, titanium alloy, tantalum, tantalum alloy,nitinol or other metals or metallic alloys. Lead 60 includes aconventional extruded outer jacket 63 with an expanded extruded innerwall defining lumen 61. Outer jacket 63 may be formed from polyurethaneor silicone, or an alloy of silicone and polyurethane. Stylet guide 62is encapsulated within lumen 61 of outer jacket 63. In this way, aconventional outer jacket 63 may be modified to incorporate stylet guide62.

The stylet guide tube 62 may be assembled into the lead by slidingstylet guide tube 62 into the lumen 61 of the outer jacket 63. Thestylet guide tube 62 may be incorporated in the lead body assemblyduring the extrusion forming process of the outer jacket 63. Anotheroption would be to insert mold the stylet guide tube 62 into the outerjacket, using a suitable mold incorporating core pins used to formlumens 65A, 65B, or other lumens in the examples of FIGS. 4B-4E. Coiledwire stylet guide 62 eases insertion and withdrawal of a stylet fromlumen 61 and may enhance steerability of lead 60. In addition, styletguide 62 allows lead 60 to maintain a substantially circular crosssection during bending to resist bi-lateral collapse or kinking.

Outer jacket 63 also forms a first conductor lumen 65A and a secondconductor lumen 65B through which conductors 64 may pass axially to lead60. In the illustrated embodiment, first lumen 65A includes four ofconductors 64 and second lumen 65B also includes four of conductors 64.Conductors 64 are positioned axially, rather than coiled, along thelength of outer jacket 63 of lead 60. Outer jacket 63 may comprise a lowdurometer material to decrease the stiffness of lead 60. For example,outer jacket 63 may comprise a polyurethane or silicone material, or analloy of silicone and polyurethane. Conductors 64 may include BSW cableto increase flexibility of lead 60.

FIG. 4B illustrates another lead 66 comprising a coiled wire styletguide 68. In the illustrated embodiment, a passive insulated metal wireforms coiled wire stylet guide 68. For example, stylet guide 68 maycomprise a coiled silver core wire coated with urethane insulation. Forexample, stylet guide 68 may include an MP25N wire. Lead 66 includes anextruded outer jacket 69 flowed to contact the coating of stylet guide68. Stylet guide 68 forms a lumen 67 that receives a stylet, whichsteers lead 66 to a therapy delivery position. Stylet guide 68 alsoincreases a resistance of lead 60 to collapse during compression byforcing a larger bend radius.

Outer jacket 69 also forms a first conductor lumen 71A and a secondconductor lumen 71B through which conductors 70 may pass axially to lead66. In the illustrated embodiment, first lumen 71A includes four ofconductors 70 and second lumen 71B also includes four of conductors 70,all of which are axially oriented along the length of lead 66. Again, asin the example of FIG. 4A, outer jacket 63 may comprise a low durometermaterial, such as polyurethane, to increase stretchability of lead 66.Conductors 70 may comprise BSW to increase flexibility of lead 66.

FIG. 4C illustrates another lead 72 comprising a coiled wire styletguide 74 with axially positioned conductors. In the illustratedembodiment, a passive metal wire coated with an insulation materialforms coiled wire stylet guide 74. As in the example of FIG. 4B, styletguide 74 may comprise a MP25N wire coated with urethane insulation. Lead72 includes an extruded outer jacket 75 flowed to contact the insulativecoating of stylet guide 74. Stylet guide 74 forms a lumen 73 thatreceives a stylet. Outer jacket 75 forms four conductor lumens 77through which conductors 76 may pass axially along the length of lead72. In the illustrated embodiment, each of conductor lumens 77 includestwo of conductors 76. Again, outer jacket 75 may comprise a polyurethanematerial with a low durometer, while conductors 76 may comprise BSW toincrease flexibility of lead 72.

FIG. 4D illustrates another lead 78 comprising a coiled wire styletguide 80 with axially positioned conductors. In the illustratedembodiment, a passive metal wire coated with an insulation forms coiledwire stylet guide 80. For example, stylet guide 80 may comprise a MP25Nwire coated with urethane insulation. Lead 78 includes an extruded outerjacket 71 flowed to the coating of stylet guide 80. Stylet guide 80forms a lumen 79 that receives a stylet. In the example of FIG. 4D,outer jacket 81 forms a single conductor lumen 83 through which alleight of conductors 82 may pass axially to lead 78.

FIG. 4E illustrates another lead 86 comprising a floating coiled wirestylet guide 88 and axially positioned conductors. In the illustratedembodiment, lead 86 includes an outer jacket 89 that forms a lumen 90,which receives stylet guide 88. Conductors 91 are positioned betweenouter jacket 89 and stylet guide 88. Conductors 91 may comprise flexibleBSW. Neither stylet guide 88 nor conductors 91 are anchored within lumen90 of outer jacket 89. Instead, outer jacket 89 contains stylet guide 88and conductors 91, such that the conductors are sandwiched between theouter jacket and the stylet guide. An electrically conductive ornonconductive metal wire coated with a lubricating insulation formscoiled wire stylet guide 88. For example, stylet guide 88 may comprise aMP25N wire coated with ETFE, or other materials such aspolytetrafluoroethylene (PTFE), modified PTFE, and polyimide, as well aspolyurethane, silicone, and polyester. The insulation around the coilturns in stylet guide 88 reduces abrasion with conductors 91.

FIG. 5 is a schematic diagram illustrating an exemplary cross-sectionalview of a lead 92 with axially oriented coiled wire conductors 97.Coiled wire conductors 97 are axially oriented in the sense that theyeach form a conductor that extends axially along the length of lead 92.In the example of FIG. 5, although each individual coiled wire conductor97 includes a single- or multi-filar coil, none of the conductors areactually coiled about the central axis of lead 92. Conductors 97preferably are formed in tight coils, such that each of the conductorsforms a substantially continuous cylindrical shape.

Lead 92 illustrated in FIG. 5 may be substantially similar to lead 50from FIG. 3. In the illustrated embodiment of FIG. 5, lead 92 comprisesan octad lead that includes eight conductors. In other embodiments, lead92 may comprise a quad lead including four conductors or another type oflead comprising any number of conductors.

In FIG. 5, lead 92 includes an outer jacket 93 that forms a lumen 96,which receives a stylet guide tube 94. Stylet guide tube 94 may besubstantially similar to coiled wire stylet guide 46 (FIG. 2) and coiledwire stylet guide 56 (FFIG. 3). In other cases, stylet guide tube 94 maycomprise a conventional plastic stylet guide tube. Stylet guide tube 94forms a lumen 95 that receives a stylet. Conductors 97 are positionedbetween outer jacket 93 and stylet guide 94, at different angularpositions about the central axis of lead 92. Hence, conductors 97 extendalong the length of lead 92 substantially parallel to the center axisdefined by outer jacket 93. Yet, each axially oriented conductor 97 isformed by a single- or multi-filar coil.

In some embodiments, as described herein, lead 92 provides enhancedstretchability to prevent lead failure, axial migration, anchor damage,and/or tissue damage at anchor points during typical patient movement.Outer jacket 93 may comprise a low durometer material to decrease thestiffness of lead 92. For example, outerjacket 93 may comprise apolyurethane or silicone material, or an alloy of silicone andpolyurethane.

Conductors 97 may comprise one or more BSW cables that provide increasedflexibility. The stranded wire used to create the BSW cables forconductors 97 may comprise a silver core. As an example, the strandedwire may comprise MP25N™ alloy, which is a biocompatible, nonmagnetic,nickel-cobalt-chromium-molybdenum alloy with high strength and corrosionresistance, with a silver core to improve conductance. In other cases,conductors 97 may comprise platinum iridium (PtIr) wires or tantalumtungsten (TaW) wires.

Conductors 97 may be constructed as BSW cables wound into a helix.Coiling or helically winding conductors 97 allows the conductors toelongate or stretch as lead 92 experiences axial loading forces duringuse. Helically wound conductors 97 may provide desirable axialcompliance as well as desirable bend-flex fatigue life. However, in acase of severe buckling, some of the helically wound conductors 97 maycollapse, causing cross-sectional flattening and concentrating thecoiled wires into a small bend radius. Furthermore, conductors 97 mayover-extend longitudinally during lead stretching, causing permanentdeformation of the coiled wires.

To address these problems, conductors 97 are coiled around fibers 98.Each of conductors 97 defines a lumen which receives fiber 98. Fiber 98may comprise a composite that includes materials such as fluoropolymer,modified fluoropolymer, polyester, nylon, liquid crystal polymer (LCP),modified LCP, ultra high molecular weight (UHMW) polyethylene, orKevlar® fiber. Kevlar® fiber is commercially available from DuPont. Ingeneral, fiber 98 provides the coiled wire with structural integrity andlimits displacement of conductor 97 along the length of lead 92.

Fiber 98 prevents bilateral collapse of the coiled wire during buckling.More specifically, fiber 98 substantially reduces an amount ofcross-sectional flattening and forces a larger bend radius. When lead 92is in use, the coiled wire of conductor 97 may stretch when a patientmoves. Fiber 98 comprises a material composite that allows fiber 98 toelongate along with conductor 97. However, fiber 98 also limits an axialstiffness and extension of conductor 97 to prevent over-extension ofconductors 97 due to axial loading. In addition, fiber 98 preferably iselastic, so that the fiber 98 returns to its original length uponrelease of the axial loading. In this way, fiber 98 ensures that thecoiled wire of conductor 97 is not over-extended, and fully recoversafter reaching a maximum axial extension.

For example, the coiled wire of conductor 97 has an axial stiffness ofno greater than 5.0 pounds/inch/inch (0.35 kg/cm/cm), more preferablybetween approximately 5.0 pounds/inch/inch and 1.5 pounds/inch/inch(0.105 kg/cm/cm), and even more preferably between approximately 3.3pounds/inch/inch (0.23 kg/cm/cm) and 1.5 pounds/inch/inch. Fiber 98limits the axial stiffness of the coiled wire of conductor 97 to no lessthan approximately 1.5 pounds/inch/inch (0.105 kg/cm/cm).

In the illustrated embodiment, each of conductors 97 comprises asingle-filar coil. Each coiled wire connects a tissue-stimulatingelectrode on a distal end of lead 92 and an electrical contact on aproximal end of lead 92. In this case, lead 92 includes eight conductors97 that each couple to an electrode. In other embodiments, lead 92 mayinclude conductors that comprise one or more multi-filar coils. Forexample, four conductors may be coiled into a single multi-filar coil.In this way, lead 92 may include eight electrodes, but carry only twomulti-filar coils within lumen 96 of outer jacket 93.

Neither stylet guide tube 94 nor conductors 97 need to be anchoredwithin lumen 96 of outerjacket 93. Instead, outer jacket 93 containsstylet guide tube 94 and conductors 97, such that the conductors aresandwiched between outer jacket 93 and stylet guide tube 94. Fibers 98comprise distal ends and proximal ends. In some cases, the distal endsof each of fibers 98 may be attached to the distal end of lead 92. Inother cases, the proximal ends of each of fibers 98 may be attached tothe proximal end of lead 92. Furthermore, fibers 98 may be attached toboth the distal and the proximal ends of lead 92. In other embodiments,where fibers 98 are not attached to lead 92, conductors 97 aresubstantially free to float within lumen 96 of outer jacket 93.

Attachment of proximal and distal ends of fibers 98 to lead 92, incombination with limitations on the axial stretchability of the fibers98, can ensure that lead 92 does not over-stretch coiled conductors 97.In this manner, fibers 98 can provide a stretch-limit for lead 92 thatprevents damage to coiled conductors 97. Although fibers 98 are disposedwithin lumens defined by coiled conductors 97, one or more fibersalternatively or additionally may be formed elsewhere within lead 92 tolimit extension of the overall lead. For example, one of more fibers 98may be placed between outer jacket 93 and stylet guide tube 94, andextend axially along the length of lead 92. In this case, each fiber 98may be coupled to outer jacket 93, stylet guide tube 94, or both tolimit extension of lead 92. Each fiber 98 may be coupled, e.g., atproximal and distal ends, to outer jacket 93, stylet guide tube 94, orboth.

In some embodiments, the diameter of the lumen defined by each coiledwire conductor 97 may vary over the length of lead 92. For example, acoiled wire conductor 97 may present a larger diameter alongsubstantially all of the lead 92, but a reduced diameter adjacent adistal tip of the lead so that the lead is more flexible in the regionin which electrodes are positioned. The outer diameter of coiled wireconductor 97 contributes to the outer diameter of lead 92. Hence, thediameter of coiled wire conductor 97 may change along the length of lead92 so that the outer diameter of the lead transitions from a larger,more extensible lead body to a smaller, more flexible distal electrodeend.

FIG. 6 is a schematic diagram illustrating a cutaway view of coiled wireconductor 97 from lead 92 of FIG. 5. Conductor 97 coils around fiber 98to create a coil 99. In some embodiments, an insulative outer member maybe included around coil 99. In this way, conductor 97 may be redundantlyinsulated not only with direct urethane insulation, but also by theinsulative outer member. The insulative outer member may reduce abrasionwith stylet guide tube 94 and other conductors within lumen 96 ofouterjacket 93. The insulative outer member may comprise a low durometermaterial, such as a polyurethane or silicone material, or an alloy ofsilicone and polyurethane.

Coil 99 of conductor 97 may comprise an external diameter ofapproximately 0.004 to 0.021 inches (0.01 to 0.053 cm), more preferablyapproximately 0.004 to 0.016 inches (0.01 to 0.04 cm), and even morepreferably approximately 0.006 to 0.015 inches (0.015 to 0.038 cm).Fiber 98 may comprise an external diameter of approximately 0.002 to0.015 inches (0.005 to 0.038 cm), more preferably approximately 0.002 to0.010 inches (0.005 to 0.025 cm), and even more preferably approximately0.005 to 0.007 inches (0.013 to 0.018 cm).

The outer diameter of coil 99 may depend on the number of conductorsincluded in coil 99. In addition, the distance between adjacent turns incoils (i.e., the pitch) may also depend on the number of conductorsincluded in coil 99. For example, a single-filar coil may comprise apitch of approximately 0.002 to 0.015 inches (0.005 to 0.038 cm). Amulti-filar coil may comprise a pitch of approximately 0.003 to 0.025inches (0.008 to 0.064 cm). Any number of conductors may be coiledaround fiber 98 as long as the multi-filar coil maintains an outerdiameter small enough to fit between stylet guide tube 94 and outerjacket 93.

FIG. 7 is a schematic diagram illustrating a cutaway view of anotherimplantable stretchable medical lead 100 for use with an IMD accordingto an embodiment of the invention. Lead 100 may be percutaneouslyimplanted using a stimulation lead introducing kit substantially similarto kit 10 illustrated in FIG. 1. Lead 100 includes an outer jacket 101,a helical reinforcement 102 and conductors 108 coiled about thereinforcement. Helical reinforcement 102 includes a raised acme thread104 with an embedded reinforcement wire 105. Alternatively, in someembodiments, thread 104 may have a trapezoidal cross-section. Forillustrative purposes, stylet 106 is also shown, but is not part of lead100 itself. Conductors 108 wrap around helical reinforcement 102 insubstantial alignment with the raised acme thread 104. In particular,raised acme thread 104 defines a helical trough or channel betweenadjacent turns to accommodate conductors 108.

Lead 100 differs from lead 40 shown in FIG. 2 and lead 50 shown in FIG.3 in that lead 100 does not comprise a separate stylet guide tube. Thebody of helical reinforcement 102 has a one-piece design that includes asubstantially cylindrical tube and helical thread 104 on the outersurface of the tube. The body of helical reinforcement 102 may consistof machined or extruded urethane, for example, such that acme thread 104is integrally formed with the cylindrical tube or is wound onto surfaceof the cylindrical tube and bonded in place. Stylet 106 fits inside alumen 103 formed from the cylindrical shape of helical reinforcement102.

As discussed previously, e.g., in the description of FIG. 1, reducingthe axial stiffness of a medical lead may provide a variety of benefits.The embodiment of the invention depicted in FIG. 7 may have a relativelylow axial stiffness. For example, the body of helical reinforcement 102may consist of urethane or other materials having a low modulus ofelasticity, providing increased stretchability. Likewise, outer jacket101 may also consist of urethane having a low modulus of elasticity.

Helical reinforcement wire 105 and conductors 108 do not experiencesignificant axial strain as lead 100 experiences strain from patientmovement, because helical reinforcement wire 105 and conductors 108 arehelically wrapped around the cylindrical shape of helical reinforcement102. As lead 100 experiences strain, the helical reinforcement 102 isdeformed, reducing the diameter of the cylindrical shape of helicalreinforcement 102, which allows the coils of conductors 108 andreinforcement wire 105 to extend under relatively low forces, withoutexperiencing significant axial tension.

Reducing the axial stiffness of a medical lead can also increase thelead vulnerability to flex fatigue, buckling fatigue, kinking, andcrush. Each of these circumstances may result in increased conductorresistivity or even conductor failure. However, stretchable medical lead100 may not only have a relatively low modulus of elasticity, but itsdesign may also reduce conductor failure due to flex fatigue, bucklingfatigue, kinking, and crush.

Helical reinforcement 102, including reinforcement wire 105 embedded inacme thread 104, may generally improve the structural integrity of lead100. For example, helical reinforcement 102 may provide protectionagainst kinking of lead 100 and bi-lateral collapse of helicalreinforcement 102. The helical shape of reinforcement wire 105 resistsbilateral collapse, buckling fatigue, flex fatigue, crush, and kinking,and reinforcement wire 105 provides structural support for lead 100. Useof a reinforcement wire, as described herein, may provide a very durableconstruction, particularly for a small profile lead, and supports axialcompliance that may help compensate for implant technique error andallow for greater patient comfort.

Reinforcement wire 105 may be formed from metallic alloys such asMP25-N, stainless steel, titanium, titanium alloy, tantalum, tantalumalloy, nitinol or other metals or metallic alloys. Further, wire 105 maybe redundantly insulated not only by acme thread 104, but also directlywith urethane insulation. While reinforcement wire 105 does not carry acurrent, insulating reinforcement wire 105 may decrease the chance thatreinforcement wire 105 would propagate a short among conductors 108.

The outer surface of acme thread 104 may touch the inner surface ofouter jacket 101, but acme thread 104 is not otherwise attached to outerjacket 101. In this manner, conductors 108 fit in the helicaltrough-like space formed between helical reinforcement 102 and outerjacket 101. This may prevent conductors 108 from bunching or kinkingwithin lead 100, even if lead 100 experiences repeated elongation andcontraction caused by patient movement. In addition, conductors 108 maynot overlap acme thread 104. While thread 104 is illustrated as an acmethread in the example of FIG. 7, other threads may also be used.

FIG. 8 is a schematic diagram illustrating a cutaway view of anotherimplantable stretchable medical lead 110 for use with an implantablemedical device according to an embodiment of the invention. Lead 110 maybe percutaneously implanted using a stimulation lead introducing kitsubstantially similar to kit 10 illustrated in FIG. 1. Lead 110 includesan outer jacket 112, a helical reinforcement wire 116, conductors 114,and a stylet guide tube 118. For illustrative purposes, stylet 120 isalso shown inserted through lumen 119 formed by stylet guide tube 118,but is not part of lead 110 itself. Conductors 114 wrap around styletguide tube 118 in substantial alignment with helical reinforcement wire116.

Lead 110 functions in a substantially similar manner to the embodimentof the invention depicted in FIG. 7. Consequently, lead 110 has a lowaxial stiffness for the same general reasons lead 100 of FIG. 7 has alow axial stiffness. As opposed to lead 100 of FIG. 7, lead 110 includesa separate stylet guide tube 118. Stylet guide tube 118 is formed by anelectrically inactive (or active) coiled flat wire, for example, asdescribed with reference to FIG. 2. In other embodiments of theinvention, different stylet guide tubes may be used.

As lead 110 experiences axial strain, the coils of stylet guide tube 118separate under relatively low stresses, but the cylindrical shape ofstylet guide tube 118 is maintained to provide structural support.Stylet guide tube 118 is coiled in an opposite direction of helicalreinforcement wire 116 and conductors 114. This may prevent helicalreinforcement wire 116 and conductors 114 from being pinched by coils ofstylet guide tube 118. In addition, conductors 114 may not overlaphelical reinforcement wire 116 and helical reinforcement wire 116 maynot overlap conductors 114.

Helical reinforcement wire 116 includes an insulated metallic wire 117embedded for structural support. For example, helical reinforcement wire116 may provide protection against kinking and bi-lateral collapse oflead 110. Helical reinforcement wire 116 includes insulated metallicwire 117, which may be formed from metallic alloys such as MP25-N,stainless steel, titanium, titanium alloy, tantalum, tantalum alloy,nitinol or other metals or metallic alloys. While insulated metallicwire 117 may not carry a current, insulation may decrease the chancethat wire 117 would propagate a short among conductors 114.

Helical reinforcement wire 116 has a rectangular cross section and maybe formed from polyurethane, polysulfone, polypropylene or PEEK. Theouter surface of helical reinforcement wire 116 may touch the innersurface of outer jacket 112. In this manner, conductors 114 fit in ahelical space formed between stylet guide tube 118 and outer jacket 112.This may prevent conductors 114 from bunching or kinking within lead110, even if lead 110 experiences repeated elongation and contractioncaused by patient movement.

FIG. 9 is a schematic diagram illustrating a cutaway view of anotherimplantable stretchable medical lead 140 for use with an implantablemedical device according to an embodiment of the invention. Lead 140includes an outer jacket 142, a helical reinforcement wire 148,conductors 146 and a stylet guide tube 150. For illustrative purposes,stylet 152 is also shown inserted through lumen 151 formed by styletguide tube 150. Conductors 146 wrap around stylet guide tube 150 insubstantial alignment with helical reinforcement wire 148, whichincludes an embedded wire 149.

Lead 140 is the same as lead 110 in FIG. 8 except that outer jacket 142includes a coiled wire 143. Coiled wire in outer jacket 142 functions ina similar manner to a wire in a common vacuum cleaner hose. Inparticular, coiled wire 143 provides structural support to outer jacket142 while allowing lead 140 to have sufficient flexibility. As lead 140experiences axial strain, outer jacket 142 elongates under relativelylow stresses, but continues to provide structural support to resistbilateral collapse and kinking. Furthermore, the coils of stylet guidetube 150 separate under relatively low stresses, but the cylindricalshape of stylet guide tube 150 is maintained to provide structuralsupport. Stylet guide tube 150 is coiled in an opposite direction ofhelical reinforcement wire 148 and conductors 146. In the illustratedembodiments, embedded coiled wire 143 within outer jacket 142 is coiledin the same direction as stylet guide tube 150. In other embodiments,embedded coiled wire 143 may be coiled in an opposite direction ofstylet guide tube 150.

Embedded coiled wire 143 may be sandwiched between two thin jacketextrusions. For example, outer jacket 142 comprises an inner layer andan outer layer. The inner layer and the outer layer may both compriseurethane. In other cases, the inner layer and the outer layer may bothcomprise silicone. Coiled wire 143 is embedded in outer jacket 142,between the outer layer and the inner layer. Coiled wire 143 may beformed from metallic alloys such as MP25-N, stainless steel, titanium,titanium alloy, tantalum, tantalum alloy, nitinol or other metals ormetallic alloys. Coiled wire 143 may be insulated redundantly by boththe inner and outer layer and with a direct insulative coating on thewire 143. While reinforcement wire 143 may not carry a current,insulating wire 143 may decrease the chance that wire 143 couldpropagate a short among conductors 146.

Placing a coiled wire 143 inside outer jacket 142 will significantlyimprove column strength and kink resistance. As an example, a 2 to 3 milwire may be wound around a thin walled inner layer with a large pitchangle, and then an outer layer is extruded over the wire and the innerjacket, thereby producing a composite jacket with a wire reinforcement.

In different embodiments of the invention, an outer jacket that includesan embedded coiled wire, similar to outer jacket 142, may be used withany internal structure of a lead. For example, an outer jacket similarto outer jacket 142 may be used with a lead comprising axial conductors,rather than helical conductors. In some cases, an outer jacket similarto outer jacket 142 may be used with a lead comprising a helicalreinforcement as shown in FIG. 7. Similarly, a lead having an outerjacket 142 with an embedded coiled wire 143 may not include a helicalreinforcement wire 148 or a coiled wire stylet guide tube 150 as shownin FIG. 9.

FIG. 10 is a schematic diagram illustrating an implantable medicaldevice (IMD) 160 for delivering electrical stimulation pulses to apatient. In the example of FIG. 10, IMD 160 includes an IMD housing 161,leads 162A, 162B, and a connector bock 163. Leads 162A, 162B each have aproximal end carrying a set of electrical contacts for connection toreciprocal electrical contacts within connector block 163, and a distalend carrying a set of electrical stimulation electrodes 164A, 164B,respectively. Although two leads 162A, 162B with eight electrodes 164A,164B each are shown in FIG. 10, a lesser or greater number of leads orelectrodes may be used in other embodiments. In general, leads 162A,162B may be constructed according to any of the embodiments describedherein, such that the leads exhibit reduced axial stiffness that permitsa degree of stretching when implanted within a patient.

FIG. 11 is a block diagram illustrating components within the IMDhousing 161 of FIG. 10. As shown in FIG. 11, IMD housing 161 may includea processor 166, memory 168, telemetry module 170, power source 172,stimulation pulse generator 174, and switch matrix 176. Processor 166executes instructions stored in memory 168 to control telemetry module170, stimulation pulse generator 174, and switch matrix 176. Inparticular, processor 166 controls telemetry module 170 to exchangeinformation with an external programmer by wireless telemetry. Processor166 specifies stimulation parameters, such as amplitude, pulse, widthand pulse rate, for use by stimulation pulse generator 174 in thegeneration of stimulation pulses for delivery to a patient. Differentstimulation parameters may be stored in memory 168 as programs orparameter sets.

The pulses may be delivered via switch matrix 176 and conductors carriedby leads 162 and coupled to respective electrodes 164. Processor 166controls switch matrix to select particular combinations of electrodes164 for delivery of stimulation pulses generated by stimulation pulsegenerator 174. For example, electrodes 164 may be combined in variousbipolar or multi-polar combinations to deliver stimulation energy toselected sites, such as nerve sites adjacent the spinal column, pelvicfloor nerve sites, or cranial nerve sites. The stimulation energygenerated by stimulation pulse generator 174 may be formulated asneurostimulation energy, e.g., for treatment of any of a variety ofneurological disorders, or disorders influenced by patient neurologicalresponse. Alternatively, in other embodiments, stimulation pulsegenerator 174 could be configured to generate cardiac pacing pulses, orcardioversion/defibrillation shocks.

Power source 172 may take the form of a small, rechargeable ornon-rechargeable battery, or an inductive power interface thattranscutaneously receives inductively coupled energy. In the case of arechargeable battery, power source 172 similarly may include aninductive power interface for transcutaneous transfer of recharge power.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A medical lead for use with an implantable medical device, themedical lead comprising: a lead body having a proximal end and a distalend; one or more electrodes at the distal end of the lead body; one ormore electrical contacts at the proximal end of the lead body; and oneor more conductors electrically coupling the electrodes and theelectrical contacts, wherein lead body has an axial stiffness of nogreater than approximately 0.35 kg/cm/cm.
 2. The medical lead of claim1, wherein the medical lead has an axial stiffness within a range ofapproximately 0.105 kg/cm/cm to 0.35 kg/cm/cm.
 3. The medical lead ofclaim 1, wherein the medical lead has an axial stiffness within a rangeof approximately 0.105 kg/cm/cm to 0.23 kg/cm/cm.
 4. The medical lead ofclaim 1, wherein the medical lead has an axial stiffness of no greaterthan approximately 0.35 kg/cm/cm in the presence of elastic deformationunder an axial elongation of approximately ten percent to approximatelythirty percent.
 5. The medical lead of claim 1, wherein the lead bodyincludes an outer jacket and a stylet guide tube, and the conductors aredisposed between the outer jacket and the stylet guide tube.
 6. Themedical lead of claim 5, wherein the outerjacket comprises one of apolyurethane material or a silicone material.
 7. The medical lead ofclaim 5, wherein each of the conductors comprises insulated braidedsilver cored wire.
 8. The medical lead of claim 1, wherein each of theconductors is formed of a material selected from the group consisting ofstainless steel, nickel-cobalt-chromium-molybdenum alloy, titanium, andnitinol.
 9. The medical lead of claim 1, wherein each of the conductorscomprises a multi-filar coil.
 10. The medical lead of claim 1, whereinthe medical lead has a length of approximately 30 to 36 centimeters. 11.The medical lead of claim 1, wherein the medical lead has a length ofapproximately 33 centimeters.
 12. The medical lead of claim 1, whereinthe lead body includes: an outerjacket; and a helical reinforcementwithin the outer jacket, wherein the helical reinforcement comprises atubular outer surface and a raised helical thread that rises from thetubular outer surface, wherein the conductors are wound around thehelical reinforcement in a helical channel defined by the raised helicalthread.
 13. The medical lead of claim 12, further comprising areinforcement wire embedded within the raised helical thread.
 14. Themedical lead of claim 1, wherein the lead body includes: an outerjacket; and a stylet guide tube inside the outer jacket, wherein thestylet guide tube comprises a coiled wire, wherein the conductors aredisposed between the outer jacket and the stylet guide tube.
 15. Themedical lead of claim 14, wherein the stylet guide tube comprises aninsulated metal wire.
 16. The medical lead of claim 14, furthercomprising a helical reinforcement wound around the stylet guide tubeand inside the outer jacket, wherein the conductors are wound around thestylet guide tube in a channel defined by the helical reinforcement. 17.The medical lead of claim 16, wherein the coiled wire of the styletguide tube is coiled in a first direction and the conductors are woundin a second direction different from the first direction.
 18. Themedical lead of claim 14, further comprising a stylet disposed withinthe stylet guide tube.
 19. The medical lead of claim 1, furthercomprising: an outer jacket, wherein the outer jacket comprises a firstinsulative layer, a second insulative layer inside the first insulativelayer, and a coiled wire embedded between the first layer and the secondlayer of the outer jacket; and conductors inside the outer jacket. 20.An implantable medical device comprising: a housing; an implantablepulse generator, within the housing, that generates electricalstimulation pulses; and a medical lead, extending from the housing, andcomprising a lead body having a proximal end and a distal end, one ormore electrodes at the distal end of the lead body, one or moreelectrical contacts at the proximal end of the lead body, and one ormore conductors electrically coupling the electrodes and the electricalcontacts, wherein the lead body has an axial stiffness of no greaterthan approximately 0.35 kg/cm/cm.
 21. The device of claim 20, whereinthe medical lead has an axial stiffness within a range of approximately0.105 kg/cm/cm to 0.35 kg/cm/cm.
 22. The device of claim 20, wherein themedical lead has an axial stiffness within a range of approximately0.105 kg/cm/cm to 0.23 kg/cm/cm.
 23. The device of claim 20, wherein themedical lead has an axial stiffness of no greater than approximately0.35 kg/cm/cm in the presence of elastic deformation under an axialelongation of approximately ten percent to approximately thirty percent.24. The device of claim 20, wherein the lead body includes an outerjacket and a stylet guide tube, and the conductors are disposed betweenthe outer jacket and the stylet guide tube.
 25. The device of claim 20,wherein each of the conductors is formed of a material selected from thegroup consisting of stainless steel, nickel-cobalt-chromium-molybdenumalloy, titanium, and nitinol.
 26. The device of claim 20, wherein eachof the conductors comprises a multi-filar coil.
 27. The device of claim20, wherein the medical lead has a length of approximately 30 to 36centimeters.
 28. The device of claim 20, wherein the medical lead has alength of approximately 33 centimeters.
 29. The device of claim 20,wherein the lead body includes: an outerjacket; and a helicalreinforcement within the outerjacket, wherein the helical reinforcementcomprises a tubular outer surface and a raised helical thread that risesfrom the tubular outer surface, wherein the conductors are wound aroundthe helical reinforcement in a helical channel defined by the raisedhelical thread.
 30. The device of claim 29, further comprising areinforcement wire embedded within the raised helical thread.
 31. Thedevice of claim 20, wherein the lead body includes: an outer jacket; anda stylet guide tube inside the outer jacket, wherein the stylet guidetube comprises a coiled wire, wherein the conductors are disposedbetween the outer jacket and the stylet guide tube.
 32. The device ofclaim 31, further comprising a helical reinforcement wound around thestylet guide tube and inside the outer jacket, wherein the conductorsare wound around the stylet guide tube in a channel defined by thehelical reinforcement.
 33. The device of claim 15, further comprising astylet disposed within the stylet guide tube.